Stabilized frequency modulator



May 24,1949. F. 2,470,892

STABILIZED FREQUENCY, monum'roa Filed July 2, 1946 MIXER FILTER FILTERCARRIER FREQUENCY OSCILLATOR OSCILLATOR MOD ULATOR OSCILLATOR OSCILLATOR2 MODULATOR INVENTOR GERARD HEPP AG T Patented May 24, 1949 I STABILIZEDFREQUENCY MODULATOR Gerard Hepp, Eindhoven, Netherlands, assignor toHartford National Bank and Trust Company, Hartford, Conn., as trusteeApplication July 2, 1946, Serial No. 680,886 In the Netherlands November25, 1942 Section 1, Public Law 690, August 8, 1946 Patent expiresNovember 25, 1962 .6 Claims. (craze-19) I I In circuit arrangements forfrequency modulating a carrier oscillation, an undesired frequencymodulation can occur due to external influences. Theseexternal'influences cause additional frequency modulation to the desiredfre- 2 understood and carried into effect it will now be described morefully with reference to the accompanying drawing, forming a part of thespecquency modulated oscillations. For example,.

variations in temperature aifect the frequencydetermining oscillatorycircuit of the carrier wave oscillator with the result that thefrequency of the generated carrier wave varies. Variations of the supplyvoltages and variations of vacuum tube characteristics, as a change inmutual conductance due to ageing of the oscillator tube,

bring about an undesired frequency modulation.

The invention has as its object, the reduction of this phenomenon.

According to the invention, the object is achieved by a circuitarrangement in which at least one of two auxiliary carrier oscillationsgenerated by separate oscillators is modulated by the modulatingoscillations. The separate oscillators employ identical construction sothat the generated frequencies are equal and are influenced to an equalextent by external influences. One of these auxiliary carrieroscillations is mixed with an oscillation of the frequency of the de- Isired carrier oscillation and the difference and sum frequencies thusobtained are mixed respectively with the other auxiliary carrieroscillation; the frequency-modulated sum or difference frequency maythen be further utilized.

The circuit arrangement according to the invention may also be such thatboth auxiliary carrier waves are modulated. In this case, the modulatingoscillations are applied to the auxiliary carrier waves in push pull. Anundesired frequency-modulation may also be developed by variations inthe frequency modulator itself. For example, if the modulator employs areactance tube, variations of the supply voltages, contact potentials,and ageing of the discharge tube are responsible for variations of thereactance presented by the tube, leading to an undesiredfrequency-modulation.

ification and in which:

Fig. 1 is a block diagram of a circuit arrangement according to theinvention,

Fig. 2 is a schematic diagram of a circuit arrangement employed inconnection with the circuit arrangement shown in Fig. l, and

Fig. 3 is a schematic diagram of. a further embodiment of the invention.

Referring to Fig. l of this drawing two oscillators I and 2 are shown,each of these generates an auxiliary carrier oscillation having thefrequency ,fm. These oscillations are frequencymodulated by modulators 3and I. The modue lating oscillations received from a source 5 aresupplied in push pull to the modulators through transformer 8. Theinstantaneous frequency changes brought about by this modulation are Afrand Air respectively. The oscillations occurring in the output circuitof the oscillator l and 2 respectively will then have the frequency 2 byan amount m. The frequency ii of the os-,

cillations in the output circuit of the oscillator i will then be:

and the frequency f: of; the oscillations in the output circuits of theoscillator 2 will be:

According to the invention, the oscillations in the output circuit ofthe oscillator tare supplied to a mixing stage 8 together with anoscillation of the frequency f0. in is the ultimately desired carrieroscillation, and is generated by oscillator I. This oscillator may becrystal controlled so that frequency fo will remain essentiallyconstant. In the output circuit of the mixing stage 8, oscillations aredeveloped having the sum-' and difference-frequency A fo+fi and \iflu-fl Of these frequencies, the diiference frequency is filtered at 9,and supplied to a second mixing stage i0, together with the modulatedauxiliary carrier oscillation received from the oscillator 2.

3 The sum frequency fs=j2+f0fl, is finally supplied to a filter l I, andfurther utilized. By substitution of Equations 1 and 2 aboveJs may bewritten in the form:

showing that the oscillation obtained is a frequency-modulated carrierwave having the desired mean frequency lo. the instantaneous desiredfrequency deviation Afl+Af2, and an un desired frequency deviationcomponent nz-m. When both oscillators i and 2 employ identicalconstruction and are influenced in a substantially identical manner byexternal conditions, as variations in temperature, etc.; nz=ni, and theundesired frequency deviation component is suppressed. Afrequency-modulated carrier oscillation of the frequency in resultswhich has only the desired frequency modulation Af1+Af2. It is evidentthat a similar result would be obtained if only one of the two auxiliarycarrier oscillations were modulated, the difference residing only inthat the expression for is, Air or Afz disappears depending on which ofthe two auxiliary carrier oscillations is modulated.

Fig. 2 shows the oscillator-modulator part of the circuit arrangementworked out in detail in accordance with the diagram of Fig. 1.

The oscillators are formed by feedback discharge tubes and 2 whosefrequency-determining oscillatory circuits l2 and I3, tuned to the samefrequency, and connected to reactance valves 3 and 4. The modulatingoscillations received from the source are fed in push-pull to thereactance tubes. The frequency-modulated auxiliary carrier oscillationsgenerated by the oscillator-modulator l, 3 and 2, 4 are fed through aconductor l4, l5 to the mixing stages 8 and I0 respectively of Fig. 1.The two mixing stages 8 and ID, the filters 8 and II and the oscillatorl are not shown in further detail in Fig. 2; the circuit arrangement ofthese component memhers is known. A circuit arrangement according to theinvention in which the two auxiliary carrier oscillations are modulatedin push-pull by the modulating oscillations has the property that anundesired frequency-modulation due to circuit unbalance is suppressed inthe modulators themselves.

If for example the supply voltages of the reactance valves 3 and 4 inthe circuit arrangement shown in Fig. 2 van. an undesired frequencymodulation mi or mz respectively of the auxiliary carrier oscillationsjm is developed. These undesired frequency modulations compensate eachother in a. manner similar to that described with regard to theundesired frequency modulation 111 and m in the oscillators I and 2 ifthe modulators are similarly constructed so that m1 and m: are similar.

An important advantage of the circuit described by the invention residesin that the operation of the circuit is dependent of the amplitude ofthe two auxiliary carrier oscillations; it is only essential that thetwo frequencies remain identical. Even if the two oscillators generatingthe auxiliary carrier oscillations are constructed in an identicalmanner, there will always be a slight difference between the twofrequencies. To reduce this difference as much as possible, use may bemade of a circuit arrangement for. automatic frequency control, forexample a circuit as shown in Fig. 3. In this circuit arrangement, theoscillations generated by the oscillators l and 2 of Fig. 1 or 2 aresupplied to limiters l6 and I1. These limiters are so adjusted thattheir outputs have equal amplitude. The output of limiter I 6 isconnected through condenser C1 to two diodes l8, l9 connected inopposition. The output of limiter I1 is connected through a condenser C2to two diodes 2D and 2| also in opposition. Each of the diodes l8 and 20has a negative bias E, supplied by the sources of voltage 22 and 23. Theconnecting point of these sources of voltage is through condenser C3 tothe junction of the diodes l9 and. 2|. It may be assumed that condenserC3 has a considerably larger magnitude than condensers Cl and C2.

The circuit arrangement of Fig. 3 operates as follows:

During the first half of the positive half-cycle of the output voltageof limiter I 6, condenser 01 is charged through diode l8. During thesec-' ond half of the positive half-cycle and the first half of thefollowing negative half-cycle, 01 becomes discharged through diode l9and condenser C3, imparting a charge to C3. This is repeated during eachsubsequent cycle, increasing the charge upon condenser C3. The charge oncondenser C3 per unit of time is proportional to the frequency of theoutput voltage of the limiter [6; thus it is proportional to thefrequency of the oscillator I. In exactly the same manner, an oppositecharge is developed across condenser C3 by the output voltage of thelimiter l1; since the output voltage of the limiter l1 charges condenserC2 through the diode 20 and C2 discharges through the diode 2i and thecondenser C3. The charge transferred thusly to condenser Ca per timeunit, is proportional to the frequency of the oscillator 2. The chargingcurrent of C:

through diode I9 is opposed to the charging current of C3 through diode2|; the condenser C3 remains without voltage when the charge receivedper time unit through the diode I9 is equal to the charge dissipated pertime unit via the diode 2|, when the frequencies of the oscillators land 2 are equal. If the oscillator frequencies differ, a positive ornegative voltage is developed across C3 as the frequency of theoscillator I differs from the frequency of the oscillator 2. Thisvoltage is supplied through conductor 24 to the modulator 4, for examplewhich controls the frequency of the oscillator 2 to conform to thefrequency of oscillator I.

What I claim is:

1. A circuit arrangement for frequency-modulating a given stableoscillation of carrier frequency, comprising means to generate a stablepilot oscillation of carrier wave frequency, means to generate a firstauxiliary oscillation of predetermined frequency, means to generate asecond auxiliary oscillation of said predetermined frequency, said firstauxiliary oscillation and said second auxiliary oscillation generatingmeans being subject to the same extraneous influences tendingundesirably to vary the frequency of said first and said secondauxiliary oscillations, means to combine a modulating oscillation withone of said auxiliary oscillations to produce a frequencymodulatedauxiliary oscillation, means to mix said pilot oscillation and saidfrequency-modulated auxiliary oscillation to produce a resultantfrequency-modulated carrier oscillation having sum and differencesideband frequencies, means to filter resultant frequency-modulatedcarrier oscillation to derive a frequency-modulated carrier oscillationhaving diflerence sideband frequencies only, means to mix said derivedresultant frequency-modulated carrier oscillation withafrequency-modulated carrier oscillation of sum and difference sidebandfrequencies, and means to filter s id frequency-modulated carrieroscillation to derive a frequency-modulated oscillation of one sidebandfrequency only whereby said first and said second auxiliary oscillationshaving undesirable frequency variations ar suppressed.

2. A circuit arrangement for frequency-modulating a given stableoscillation of carrier frequency, comprising means to generate a stablepilot oscillation of carrier frequency, means to generate a firstauxiliary oscillation of predetermined frequency, means to generate asecond auxiliary oscillation of said predetermined frequency, said firstauxiliary oscillation and said second auxiliary oscillation generatingmeans being subject to the same extraneous influences tendingundesirably to vary the frequency of said first and said secondauxiliary oscillations, means to combine a modulating oscillation withsaid first and said second auxiliary oscillations to produce a firstfrequency-modulated auxiliary oscillation of different frequency and asecond frequencymodulated auxiliary oscillation of sum frequency, meansto mix said pilot oscillation and said first frequency-modulatedauxiliary oscillation to produce a resultant frequency-modulated carrieroscillation having sum and difference sideband frequencies, means tofilter said resultant frequency-modulated carrier oscillation to derivea resultant frequency-modulated carrier oscillation having differencesideband frequencies only, means to mix said derived resultantfrequencymodulated carrier oscillation with said secondfrequency-modulated auxiliary oscillation to produce a resultantfrequency-modulated carrier oscillation having sum and differencesideband frequencies, and means to filter said resultantfrequency-modulated carrier oscillation to derive a frequency-modulatedcarrier oscillation of sum sideband frequency only whereby said firstand said second auxiliary oscillations having undesirable frequencyvariations are suppressed.

3. A circuit arrangement for frequency-modulating a given stableoscillation of carrier frequency, comprising means to generate a stablepilot oscillation of carrier frequency, means to generate a firstauxiliary oscillation of predetermined frequency, means to generate asecond auxiliary oscillation of said predetermined frequency, said firstauxiliary oscillation and said second auxiliary oscillation generatingmeans being identical in construction and subject to the same extraneousinfluences tending undesirably to vary the frequency of said first andsaid second auxiliary oscillations, means to combine a modulatingoscillation with said iliary oscillations to produce a first frequencymodulated auxiliary oscillation of difference frequency and a secondfrequency-modulated auxiliary oscillation of sum frequency, means to mixsaid pilot oscillation and said first frequencymodulated auxiliaryoscillation to produce a resultant frequency-modulated carrieroscillation having sum and diiference sideband frequencies,

means to filter said resultant frequency-modulated carrier oscillationto derive a resultant frequency-modulated carrier oscillation havingdifference sideband frequencies only,means to mix said derived resultantfrequency-modulated carrier oscillation with said secondfrequency-modulated auxiliary oscillation to produce'a resultant firstand said second aux-- tion having difference quency only whereby firstand said second predetermined frequency,

6 frequency-modulated carrier sum and difference sideband meansto-filter said resultant oscillation having frequencies, andfrequency-modiflated carrier oscillation to derive a frequencymodulatedcarrier oscillation of sum sideband fresaid first and said secondauxiliary oscillations. having undesirable frequency variations aresuppressed.

. 4. A circuit arrangement for frequency-modulating a given stableoscillation of carrier frequency, comprising means to generate a stablepilot oscillation of carrierfrequency, means to generate a firstauxiliary oscillation of predetermined frequency, means to generate asecond auxiliary oscillation of said predetermined frequency, said firstauxiliary oscillation and said second auxiliary oscillation generatingmeans being subject to the same extraneous influences tendingundesirably to vary the frequencyof said first and said second auxiliaryoscillations, an automatic frequency control circuit ,to maintain thefrequencies of said first and said second auxiliary oscillationssubstantially equal, means to combine a modulating oscillation with saidauxiliary oscillations to produce a first frequency-modulated auxiliaryoscillation of difference frequency and a second frequency-modulatedauxiliary oscillation of sum frequency, means to mix said pilotoscillation and said first frequency-modulated auxiliary oscillation toproduce a resultant frequency-modulated carrier oscillation having sumand difference sideband frequencies, frequency-modulated carrieroscillation to derive a resultant frequency-modulated carrier oscillasideband frequencies only, means to mix said derived resultantfrequency? modulated carrier oscillation with said secondfrequency-modulated auxiliary oscillation to produce a resultantfrequency-modulated carrier oscillation having sum and differencesideband frequencies, and means to filter said resultantfrequency-modulated carrier; oscillation to derive a frequency-modulatedcarrier oscillation of sum sideband frequency only whereby said firstand said second auxiliary oscillations having undesirable frequencyvariations are suppressed.

5. A circuit arrangement for frequency-modulating a given stableoscillation of carrier fr equency, comprising means to generate a stablepilot oscillation of carrier wave frequency, a first oscillator togenerate a first auxiliary oscillation of predetermined frequency, asecond oscillator to generate a second auxiliary oscillation of saidsaid first and said second oscillators being identical in constructionand being subject to the same extraneous influences tending to vary thefrequencies produced therein undesirably, a first reactance tube cir-'cuit coupled to said first oscillator, a second reactance tube circuitcoupled to said second oscillator, means to apply modulatingoscillations to said reactance tubes in push-pull relationship toproduce a first frequency-modulated auxiliary oscillation of differencefrequency and a second frequency-modulated auxiliary oscillation of sumfrequency, means to mix said pilot oscillation and said firstfrequency-modulated auxiliary oscillation to produce a resultantfrequency-modulated carrier oscillation having sum and differencesideband frequencies, means to filter said resultant frequency-modulatedcarrier oscillation to derive a resultant frequency-modulated"carrier oscillation 'having difference sideband frequencies only, means to mixsaid derived resultantfremeans to filter said resultant quency-modulatedcarrier oscillation with. said second frequency-modulated auxiliaryoscillation to. produce a resultant frequency-modulated carrieroscillation having sum and difference sideband frequencies, and means tofilter said resultant frequency modulated carrier oscillation to derivea frequency-modulated carrier oscillation of sum sideband frequency onlywhereby,

.of predetermined frequency, a second oscillator to generate a secondauxiliary oscillation of said predetermined frequency, said first andsaid second oscillators being identical in construction and beingsubject to the same extraneous influences tending to vary thefrequencies produced therein undesirably, a first reactance tube circuitcoupled to said first oscillator, a second reactance tube circuitcoupled to said second oscillator, a frequency control circuitcomprising a first amplitude limiter coupled to said first oscillator, asecond amplitude limiter coupled to said second oscillator and beingadjusted to limit the amplitude value of said second auxiliaryoscillation to the amplitude value of said first auxiliary oscillation,a first diode having the anode thereof capacitively coupled to saidfirst limiter, a second diode having the cathode thereof connected tothe anode of said first diode, a third-diode having the cathode thereofcapacitively coupled to said second limiter, a fourth diode having theanode thereof connected to the cathode of said third diode and thecathode thereof connected to the anode at said second diode and to saidsecond reactance tube circuit, means to apply negative bias to saidfirst diode, means to apply negative bias to said third diode and acapacitor coupling the cathode of said first diode and the anode of saidthird diode to the anode of said second diode and the cathode of saidfourth diode whereby the frequency of said first oscillator ismaintained substantially equal to that of said second oscillator, meansto mix said pilot oscillation and said first frequency-modulatedauxiliary oscillation to produce a resultant frequency-modulated carrieroscillation'having sum and difference sideband frequencies, means tofilter said resultant frequency-modulated carrier oscillation to derivea resultant frequency-modulated carrier oscillation having differencesideband frequencies only, means to mix said derived resultantfrequencymodulated carrier oscillation with said secondfrequency-modulated auxiliary oscillation to produce a resultantfrequency-modulated carrier oscillation having sum and differencesideband frequencies, and means, to filter said resultantfrequency-modulated carrier oscillation-to derive a frequency-modulatedcarrier oscillation of sum sideband frequency only whereby said firstand said second auxiliary oscillations having undesirable frequencyvariations are suppressed.

GERARD HEPP.

narsnancas drum The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,075,071 Usselman Mar. 30, 19372,290,159 Armstrong July 21, 1942

